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Chapter 5 Volume Management
5.2 Functions in the Action Area for Volume
The following parameters can be specified:
•
Prefetch Limit (PL)
Specify the value to calculate the amount of data that is to be read from the disk in advance
(amount of data to prefetch from the disk) when the cache detects the sequentiality of data
access.
•
Force Prefetch (FP)
Specify whether to perform prefetch even if the sequentiality of data access is not detected.
•
Multi Writeback Count (MWC)
Specify the number of processes that can be written back at the same time.
The procedure to modify cache parameters is as follows:
Procedure
1
2
•
The same cache parameter can be applied for multiple volumes in a
single operation.
•
Cache parameters cannot be specified for TPVs.
Select which volume to change the cache parameters for (multiple selections
can be made) and click [Modify Cache Parameters] in [Action].
Change the cache parameters and click the [Modify] button.
•
Prefetch Limit (PL)
Specify the Prefetch Limit of the volume between 0 and 64.
If "0" is specified for the "Prefetch Limit (PL)", data prefetch is not performed.
When a large "Prefetch Limit" value is specified, the amount of data that is read from a
disk for a single read process is increased. As a result, the number of times that data
needs to be read (Staging) from the disk is reduced. However, the amount of data that
is read from the disk for a single read process increases and requested data may not
be read before the next read or write request from the host is issued. In this case, the
waiting time for Staging may increase.
•
Force Prefetch Mode (FP)
Select whether to enable or disable the Force Prefetch Mode to forcibly read to the
cache.
This setting is required to improve the sequential read access performance.
To enable this mode, select "ON". To disable this mode, select "OFF".
When a number of sequential accesses are performed for one volume, the ETERNUS
DX Disk storage system may determine that random accesses have occurred. By
selecting "ON" for "Force Prefetch Mode", prefetching is forcibly performed in such a
case.
However, for random read access, unnecessary data is read from the disk and random
read access performance may be reduced.
•
Multi Writeback Count (MWC)
Specify the Multi Writeback Count. The Multi Writeback Count cannot be specified for
volumes that are concatenated by the LUN concatenation function.
When specifying a larger "Multi Writeback Count" value, sequential write access
performance is improved. Note that this is not effective when a number of random write
accesses occur. Depending on the ratio of read access and write access, read access
performance may be degraded.
ETERNUS Web GUI User's Guide
Copyright 2011 FUJITSU LIMITED
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